Many of the quiet and power-efficient PCs are very small, but I don't really care what the size is, so I don't want to pay extra or sacrifice performance to get something far smaller than I need.

I want a PC which I can leave on 24/7 without worrying about running costs, but which can play back audio bit-perfectly (external DAC is fine, but decent PCI sound card would be better), and play full HD content (e.g. BBC HD, which is H.264 1080i50 with MBAFF). Needs IEE1394, a way of getting DVB-S2 into it (implies another PCI slot!), and some kind of PC monitor and TV out.

Ideally, it would be able to handle video editing and complex AVIsynth scripts, but wouldn't use many Watts of power when sitting idle or doing less intensive tasks.